The present invention relates to work holders, and particularly relates to such devices using radiant energy to bond and debond a radiation responsive adhesive interposed between a workpiece and a fixture.
Fixtures are used in manufacturing practice to locate and hold the workpiece relative to the manufacturing process, assembly process, or inspection process. One of the most demanding manufacturing operations is machining, and will be used as an example. While the configuration of a machining center fixture is typically application specific, nearly all utilize mechanical elements called locators, clamps, and supports. Locators are fixed mechanical elements that are used to position the workpiece relative to the fixture base and ultimately the machining center. Clamps are mechanisms that are used to push the workpiece against a subset of locators. Clamps are typically actuated through the relative turning of a nut and screw, hydraulically, or pneumatically.
Supports are mechanisms that are used to increase the rigidity of the fixture-workpiece system. Supports differ from clamps in that supports are brought into contact with the workpiece with minimal pre-load, and subsequently locked into place. Like clamps, supports may be actuated through the relative turning of a nut and screw, hydraulically, or pneumatically. Because of imperfections in workpiece surfaces, and the necessity to locate the part using only six or less locators, supports are engaged only after the workpiece has been brought into contact with the locators and clamped. In many applications additional clamps are actuated in order to force the workpiece into contact with the supports, and thus increase the pre-load of the fixture on the workpiece.
A typical workpiece loading cycle involves the following steps. The workpiece is brought into contact with the locators. The clamps are actuated and forced into contact with the workpiece. This creates pre-loaded joints between the workpiece, clamps, and a subset of locators. If used, supports are actuated and brought into light contact with the workpiece. Supports are subsequently locked in place. Additional clamps can be actuated to hold the workpiece against a subset of supports. The workpiece and the fixture elements are essentially a single assembled structure.
During the manufacturing cycle, the workpiece is restrained by the actuated clamp forces and the frictional contact forces at the pre-loaded fixture-workpiece joints. The magnitude of the actuated clamp forces is critical. When clamp forces are too small, the workpiece may slip within the fixture during machining. When clamp forces are too large, the workpiece may excessively deform within the fixture prior to machining.
An important property of the fixture-workpiece system is its dynamic stiffness at the workpiece surfaces to be machined. High dynamic stiffness is necessary to insure that the surfaces do not vibrate excessively during the machining process, and are thus free from excessive form errors and chatter marks.
Dynamic stiffness is a direct function of the geometry and elastic modulus of the workpiece, the spatial arrangement of the fixture elements, the geometry and elastic modulus of the fixture elements, and the coefficient of friction between the workpiece and fixture elements. To a much lesser extent, it is also a function of the joint pre-load forces. An important performance measure of a machining fixture is its ability to impart high dynamic rigidity to the fixture-workpiece system while maintaining cutting tool access to the features that need to be machined.
Other important measures include workpiece load time (includes workpiece mounting and the engagement of the clamps and supports), workpiece unload time (includes disengagement of the clamps and supports, workpiece dismounting, and cleaning of debris from the locator and support contact surfaces), flexibility or ability to be reconfigured to hold different parts, and capital cost. Fixture design is very application specific since every machining application differs with respect to workpiece complexity, machined feature tolerances, required material removal rates, required cycle time, and total number of parts produced.
When actuation forces are set properly, it is very rare for workpieces to slip out of a fixture during machining. Additionally, the holding strength of these fixtures is relatively insensitive to the cleanliness of the workpiece. However, the ability of the fixture to locate the workpiece is very sensitive to cleanliness of the workpiece. Also, if automated clamping and support systems are used, workpiece load and unload time can be made relatively short.
The capital cost of a fixture utilizing this technology varies from hundreds of dollars for a precision vise to over sixty-thousand dollars for a fully automated, tombstone-fixture system. Additionally, there are costs for an external source of actuation (hydraulic fluid transmission system, nut runners, etc.).
However, there are several limitations to traditional fixturing techniques. In many applications, fixture elements cannot be placed at strategic locations on the workpiece because the location is inaccessible, impossible to pre-load through clamp actuation, too compliant, and/or will result in significant pre-load deformation of the workpiece if pre-loaded. This results in insufficient dynamic stiffness of the fixture-workpiece system. In turn this leads to problems with forced vibration and chatter during machining or a significant reduction in the material removal rate of the machining process in order to prevent it. In general this problem degrades both productivity and part quality.
Clamp actuation always leads to elastic, pre-load deformation of the workpiece. Clamping forces are rarely monitored in practice and can vary significantly from workpiece to workpiece, especially when manually actuated. In many cases, clamping forces are much larger than is necessary to hold the workpiece. In other cases, clamping forces that are minimally sufficient to hold the workpiece still result in excessive pre-load deformation of the workpiece. In a number of applications, pre-load deformation is sufficiently high by itself to cause machined feature errors to go out of tolerance. In many other cases, the deformation is sufficiently high to significantly stack up with other sources to cause the part to go out of tolerance.
In most applications, the contact area between the workpiece and fixture is very small. The high stresses that result from clamping forces and machining forces can lead to plastic indentation and/or scratching of the workpiece surface at the fixture-workpiece contact regions. This problem effects part quality and can cause the part to fail surface texture tolerances.
Fixture elements, especially clamps, typically block access to the workpiece surfaces that need to be machined. This necessitates extra set ups, which significantly increase the total, lead time for part machining and/or require the expense of extra machine tools, cutting tools, and fixturing. In addition, every set up requires the part to be located with respect to the machine tool. Since this process is always subject to bias error and random error, every additional set up increases the stack of machined feature errors, in particular orientation, position, and profile type errors.
Fixture elements, especially clamps, can obstruct tool paths to surfaces that need to be machined. This frequently results in tool crashes that damage the cutting tool, machine tool, and fixture element. It also results in substantially reduced productivity, as the cutting tool is required to rapid traverse around the fixture elements.
Fixture elements, especially clamps, can lie outside the envelope of the workpiece, and thus occupy area that could otherwise be used to hold other workpieces. This reduces the number of workpieces that are held on a base plate or tombstone. In turn, this significantly increases the per part cycle time associated with cutting tool changes, pallet changes, and workpiece to workpiece rapid traverse time.
The positions of supports must be adjusted over a very small distance (0.001 in–0.005 in.) in order to be brought into contact with the located and clamped workpiece. This requires moving components, whose use degrades the stiffness of the support.
The limitations just described become more apparent as machined feature tolerances become tighter, the geometric complexity of the workpiece increases, workpiece stiffness decreases, or workpiece hardness decreases.
For special applications, alternative fixturing techniques are available to overcome some of the limitations of conventional fixturing. These alternatives include the use of alternative forces to clamp the workpiece and adhesive bonds.
Three commercially available fixturing technologies that use alternative clamping forces that do not rely on mechanical clamps are vacuum chucks, magnetic chucks, and electro-static chucks. In all three fixture types, the clamping forces can be turned on or off instantaneously.
For example, a conventional vacuum chuck, such as models manufactured by Dunham, includes gripper plate perforated with holes/channels. The holes/channels are connected to a vacuum pump, and are opened and closed through a system of valves. When the vacuum pump is turned on, air pressure forces the part against the gripper plate. In order to maintain this vacuum, the holes and channels must be sealed off by contact between the surrounding chuck-workpiece surfaces. The magnitude of this force is the product of the sealed hole/channel area between the workpiece and the gripper plate and the atmospheric pressure (up to 12 psi).
Vacuum chucks are used to hold workpieces made from any material. However the bottom surface of the workpiece must be smooth. Furthermore holes or channels that will be exposed by the machining process must be plugged. The axial direction (or perpendicular to contact surfaces) holding strength of a vacuum chuck can not exceed atmospheric pressure (12 psi). Likewise assuming a coefficient of static friction of 0.2, which is a value typical of metal-to-metal contact, the shear direction (or parallel to contact surfaces) holding strength of a vacuum chuck is roughly 2.4 psi.
Due to their low holding strength, vacuum chucks are typically used for the light milling and drilling of small, thin thickness parts. They are also used for the high speed machining of workpieces that have very large, smooth contact surfaces. In these cases, the low holding strength of the fixture is overcome by the large contact area between the fixture and workpiece. These applications are typically found in the aerospace industry.
A conventional magnetic chuck, such as models manufactured by Tecnomagnete®, is used to hold workpieces made from ferro-magnetic materials (cast iron, steel, and some nickel alloys). The chuck generates a magnetic field either through the use of a permanent magnet or electro-magnet. In both cases, the magnetic force pulls the workpiece against the gripper plate.
The strength of the magnetic force acting on the workpiece is a direct function of the strength of the magnetic field and the proximity of the workpiece material relative to the gripper plate. The former is heavily influenced by the ferromagnetic properties of the workpiece material and the strength of the magnets. In general the stronger the field and/or the closer the material, the stronger the magnetic force. It is also known that for electromagnetic chucks, a decrease in the workpiece-gripper plate contact area or increased surface roughness of the contacting workpiece surface leads to a significant decrease in magnetic force.
Magnetic chucks are capable of exerting significantly larger clamping forces than vacuum chucks axial direction holding strengths as high as 205 psi (for low carbon steel) and shear direction holding strengths of 40.2 psi (assuming a coefficient of friction of 0.2). Consequently, magnetic chucks are used in applications involving higher material removal rates. They are mostly for grinding applications. However they are also used for machining center operations as well. Furthermore if desired, parallels can placed on top of the gripper plate in order to locate the bottom surface of the workpiece. Since this displaces the workpiece material away from the gripper plate and reduces the contact area, it also decreases the magnetic force acting on the workpiece. Workpieces held by magnetic chucks are often left with residual magnetism. This residual magnetism is eliminated or reduced with separate demagnetizing equipment.
A conventional electro-static chuck is used to hold electrically conductive materials. The gripper plate is an electrode coated with a nonconductive material such as a plastic resin. The workpiece and gripper plate are connected to a voltage source, which causes positive electric charge to be deposited on the workpiece and negative electric charge to be deposited on the gripper plate (or vise-versa). In turn this results in an electrostatic force that pushes the workpiece against the gripper plate.
In general, the electro-static force that is generated is quite small, usually less than 20 psi per workpiece-gripper plate contact area. Because of this weak clamping force, electro-static fixtures are rarely used for machining applications. However they are used extensively by the semi-conductor industry for holding semi-conductor materials for a variety of other processes.
All three technologies offer the following advantages. Greater access to the workpiece, thus permitting more surfaces to be processed in a single set up and/or a greater number of workpieces to be held in a single set up. Evenly distributed, small contact stresses between the bottom of the workpiece and the gripper plate, thus minimizing part degradation due to pre-load deformation, plastic indentation, and scratching. Instantaneous activation and deactivation of clamping forces. However because of their technical limitations, these technologies are not used for the majority of machining center applications.
Adhesive bonding is used to hold flexible and/or geometrically complex parts that cannot be mechanically clamped nor held effectively in either a vacuum chuck or magnetic chuck or electro-static chuck. In general this technique is typically restricted to the manufacture of a very small number of parts. This is due to the long lead time necessary to form an adhesive bond and to destroy (or structurally weaken) the bond once machining has been completed.
Some commercially available adhesive systems (for example those manufactured by MCP Group) utilize a low melting temperature, bonding material to either adhere the workpiece to a sub plate and/or encapsulate it. These bonding materials are either metal, polymer, or water. These metals are an alloy of bismuth, zinc, and tin. The melting points of these metals range from 75° C. to 250° C. are dependent upon their composition.
One embodiment to use these materials for simple bonding to a sub plate includes a pool mounted on to the top surface of the sub plate, and the workpiece is placed in the pool. A small gap between the bottom surface of the workpiece and the sub plate may be enforced via shim stock or through some other mechanical means. The low melt metal is heated to a liquid state, and subsequently poured into the pool to a level just above the workpiece-sub plate gap. The metal is allowed to cool and harden. The metal acts as an adhesive to bond the workpiece to the sub-plate. In addition, hardened metal surrounding the edges of the workpiece create a mechanical barrier to resist workpiece motion (i.e. partial encapsulation). At this time, the subplate is mounted to the machine tool and the exposed surfaces of the workpiece are machined. Upon completion, the workpiece is removed by either melting the bonding metal via a torch or placing the sub-plate and workpiece into an oven.
Another complete or partial encapsulation of the workpiece includes a mold. The walls of the mold are in the form of easy to grip surfaces such as parallel planar surfaces or a cylindrical surface. Molten metal, as described above, is subsequently poured into the mold and allowed to encapsulate the workpiece including its internal cavities. The encapsulated workpiece is removed from the mold and mounted into either a vise or chuck. Dismounting of the workpiece is carried out as described above.
A thermoplastic equivalent of this material is commercially available, for example Rigidax™ manufactured by M. Argüeso & Co. Inc. Various formulations of thermoplastic have melting points that range between 65° C. to 100° C. Both low melt metals and thermoplastic have been successfully used in a number of machining applications. However their use has limitations, such as thermal distortion of thin walled workpieces during solidification and mechanical distortion due to significant shrinkage of the molten substance. In addition, both have very low bonding strength with aluminum (0.246 psi for low melt metal, 9.98 psi for thermoplastic).
Another embodiment uses a system of coolant coils and heating elements to solidify a coolant by lowering the temperature of coolant below the freezing point of the coolant, similar to an ice rink. One such device is the Ice Vise™, manufactured by Horst-Witte. The device includes features very similar to a magnetic chuck with the exception that it has a very small retaining wall. It is designed to mount directly to a machine tool table. Beneath the chuck contact surface runs a system of coolant coils and heating elements.
To use the frozen vise, a thin film of water deposited on to the chuck contact surface. The workpiece is placed in contact with this surface. Coolant is subsequently driven through the coils, which causes the water film to freeze and bond the workpiece to the contact surface. The frozen vise control system continues to drive coolant through the coils in order to maintain the ice temperature around −10° C. ±2° C. The time required to freeze the film is reported to be around 90 seconds.
After machining, the heating elements are activated to melt the ice film and release the workpiece. The time required to do this is reported to be around 90 seconds. Variants of this device include those that use an integrated vacuum chuck to hold the workpiece during the freezing process, and a unit that uses shop air rather than traditional coolant for the cooling medium. The system will also drive warm shop air through the same coils for the purpose of melting the ice film.
Another embodiment has a significantly deeper retaining wall, which allows coolant, such as water or water-based gel, to be pooled around the outer surfaces of the workpiece. When frozen, the solidified fluid partially encapsulates the workpiece. The water-based gel can provide an even greater degree of encapsulation. In these cases, the gel is packed around the walls of workpiece and into accessible cavities. As the fixture and workpiece cool, the gel freezes into a solid block. The cycle times for the use of these embodiments is considerably longer than other alternatives due to the greater thermal masses involved. It has been shown that ice at −10° C. has an ultimate tensile strength ranging from 145 psi to 1300 psi, depending upon strain rate.
The limitations of a freezing work holding device is that it cannot be used in orientations other than the vertical. If vertical thru holes are to be drilled into the workpiece, a significant clearance must be established between the bottom of the workpiece and the chuck. This additional clearance must be filled with either water or water based gel, thus increasing the thermal mass that must be frozen, and dramatically increasing the freezing and melting cycle time. Lastly the freezing process will inevitably lead to severe temperature gradients within the workpiece, which will result in its thermal distortion. In turn this can lead to dimensional control problems. Even if the workpiece is thermally soaked to −10° C., this will still lead to significant dimensional problems or process development time to overcome them, since all finished part dimensions must be measured at 20° C.
Another work holding device uses a solid adhesive starting material such as Mitee-Grip™ manufactured by Mitee-Byte®. The solid adhesive is a heat-activated wax-based adhesive that is either embedded in paper, coated on nylon mesh, or pressed into stick form to hold very thin or hard-to-hold parts. The paper product, for example, can hold smooth, flat parts. The mesh product captures additional wax material in the web and aides in holding irregular shaped parts. The stick form material is used in shallow cavities for holding concave, convex and flimsy parts.
One example of a solid adhesive work holding device requires the bottom surface of the workpiece to be covered with the solid adhesive and pressed against a sub plate. The sub plate, adhesive, and workpiece are then positioned on a hot plate (or in an oven) and heated to a temperature above the melting point of the solid adhesive, for example between 80° C. and 90° C. At such a temperature, the solid adhesive melts and covers the workpiece and sub plate surfaces. The sub plate, adhesive, and workpiece cool to room temperature and become one unified body.
The next step in the process is to mount the sub plate to the machine tool in preparation for machining. After machining, the workpiece and sub plate are reheated using the procedure described above. Once the adhesive has melted, the workpiece is separated from the sub plate. This complex process and time consuming process is usually used for special machine jobs. It is believed that the tensile strength of the solid adhesive is approximately from 62 psi to 600 psi.
None of the adhesive bonding systems described thus far use adhesives that have strengths equivalent to those (3000 psi to 5000 psi) associated with permanent, high strength, structural adhesives. Structural adhesives are not used in part, because of their long cure times. Structural adhesives are cured by a variety of means, including exposure to moisture, addition of a chemical catalyst, and thermal activation. Each curing mechanism has a relatively short setting time (10 seconds to one minute), but their time to full cure is considerably longer (15 minutes to hours).
Another major limitation of structural adhesives is their inability to be re-melt once cured. In addition, their strength can only be diminished by elevating their temperatures to very high temperatures and/or exposure to harsh chemicals, neither which can be done easily in most cases without damage to the workpiece.
A solution to the curing and debonding problems is the use of radiation. Many structural adhesives can be cured through exposure to radiation. This radiation is typically either electromagnetic radiation or electron bombardment. Using these means, a structural adhesive can be fully cured in seconds. Likewise radiation can also be used to structurally weaken adhesive bonds to allow easy removal of the workpiece from the fixture.
In order for an adhesive fixturing system to utilize structural adhesives and radiation, it must be capable of exposing the adhesive between a fixture-workpiece joint to radiation to cure the adhesive within seconds, and radiation to thermally destroy or structurally weaken the adhesive between a fixture-workpiece joint within seconds with negligible thermal transfer to the workpiece and fixture. The thermal transfer requirement is critical, because thermal growth of the fixture-workpiece system can lead to significant manufactured feature errors.
Accordingly, it is an object of the present invention to provide a system and method to hold workpieces with minimum pre-load distortion, with maximum rigidity, and with maximum accessibility to the manufacturing process.
It is another object of the invention to provide a system and method applicable to low volume, job shop applications as well as dedicated high volume applications
It is an alternative object of the invention to provide a system and method to significantly reduce the lead time and cost for part manufacture while simultaneously improve part quality.